Astronomy in the radio, X-ray,
and gamma ray and ultrahigh energy gamma ray frequency bands is made
possible largely by relativistic particles, which emit at these
frequencies when their paths are bent by magnietic fields. The Crab
Nebula is but one famous example of synchrotron radio, X-rays and
gamma rays being emitted by electrons moving at 0.999999 of the speed
of light. We attempt to understand how, and under what circumstances,
violent explosions accelerate a small minority of particles to such
enormous energies.